manner fits it for reception into the
lungs.] and the other nitrogen. There is another gas usually found with
these two, in smaller quantity, called carbonic acid gas; but whether it
is necessary, in a very small quantity, to health, chemists, I believe,
are not agreed. One thing, however, is certain--that if any portion of
it is healthful, it must be very little--not more, certainly, than
one-fiftieth or one-hundredth of the whole mass.

It is by means of the oxygen it contains, that air sustains life and
combustion. Were it not for this, neither fires nor candles would burn,
and no animal could breathe a single moment. Breathing consumes this
oxygen of the air very rapidly. When the oxygen is present in about a
certain proportion, combustion and respiration go on well, but when its
natural proportion is diminished, the fire does not burn so well,
neither does the candle; and no one can breathe so freely.

Not only are breathing and combustion impeded or disturbed by the
diminution of oxygen in the atmosphere, but just in proportion as oxygen
is diminished by these two processes, or either of them, carbonic acid
is formed, which is not only bad for combustion, but much worse for
health. If any considerable quantity of it is inhaled, it appears to be
an absolute poison to the human system; and if in _very large quantity_,
will often cause immediate death.

It is this gas, accumulated in large quantities, that destroys so many
people in close rooms, where there is no chimney, nor any other place
for the bad air to escape. But it not only kills people outright--it
partly kills, that is, it poisons, more or less, hundreds of thousands.

In a nursery there is the mother and child, and perhaps the nurse, to
render the air impure by breathing, the fire and the lamp or candle to
contribute to the same result, besides several other causes not yet
mentioned. One of these is nearly related to the former. I allude to the
fact that our skins, by perspiration and by other means, are a source of
much impurity to the atmosphere; a fact which will be more fully
explained and illustrated in the chapter on Bathing and Cleanliness. It
is only necessary to say, in this place, that it is not the matter of
perspiration alone which, issuing from the skin, renders the air
impure; there are other exhalations more or less constantly going off
from every living body, especially from the lungs; and carbonic acid gas
is even formed all over the